Resonant converters such as LLC resonant converters are widely used in bus converters where no stringent load regulation is required. The resonant converter typically works with open loop control, and is designed to operate at a switching frequency fs as following equation (1) for best efficiency performance.
                              f          s                =                  1                      2            ⨯            Π            ⨯                          (                                                Lr                  ⨯                  Cr                                +                Tdead                            )                                                          (        1        )            wherein Lr represents the inductance of a leakage inductor, Cr represents the capacitance of a resonant capacitor, Tdead represents the dead time between a high-side power switch and a low-side power switch in a power stage.
This switching frequency works well under heavy load condition. However, the leakage inductor and the resonant capacitor in real applications may deviate far from their preset values, causing the resonant converter to deviate from its best efficiency point.